This invention relates generally to gas turbine engines, and more particularly, to methods and apparatus for operating gas turbine engines.
Gas turbine engines typically include high and low pressure compressors, a combustor, and at least one turbine. The compressors compress air which is mixed with fuel and channeled to the combustor. The mixture is then ignited for generating hot combustion gases, and the combustion gases are channeled to the turbine which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to propel an aircraft in flight or to power a load, such as an electrical generator.
Because gas turbine engines must be capable of operating in a plurality of operating conditions, stable burning is essential for engine operation over a wide range of engine operating conditions. More specifically, stable combustion facilitates reducing engine blowout while achieving engine rated thrust or power levels. Furthermore, stable combustion also facilitates reducing engine screech, rumble, or howl. Screech is characterized by high pressure acoustic oscillations at a frequency above 300 Hz., and may be caused by a coupling/feedback mechanism of the combustion process with a natural acoustic transverse mode (radial and tangential) of a combustion chamber defined within the combustor. Rumble or howl is also characterized by high pressure acoustic oscillations, but at frequencies below 300 Hz. More specifically, at such frequencies, combustion instability may be caused by a coupling/feedback mechanism of the combustion process with a natural axial mode of the combustion system. Continued operation with screech, rumble, or howl may cause hardware damage to occur.
To facilitate reducing potentially harmful combustion resonance, at least some known combustors have been modified with extensive and expensive design changes. Such design changes may include the addition of acoustic suppressors that are tuned to facilitate reducing resonant frequencies. Frequent maintenance may occur if a combustion instability persists in a product introduced in the field. Additionally, damage to fuel nozzles, liners, and other combustor components including suppressors may occur with continued operation during combustion instability.
Other known combustors include complex active combustion control systems (ACC) that include a pulsator coupled upstream from a controller that is coupled between the pulsator and the fuel manifold. The pulsator pulses the fuel flow to the fuel manifold at a resonant frequency to enhance combustion stability. The controller receives continuous feedback from the combustor and times the fuel pulsation such that the fuel flow increases at the low portions of the oscillation and decreases at high portions of the oscillation, such that the system serves as a wave cancellation. However, because the controller is downstream from the pulsator, establishing the accurate timing of the controller with respect to the pulsator may be difficult. Furthermore, such systems may provide only limited benefits when spinning tangential modes instead of merely standing acoustic modes are present during engine operations. Moreover, during such conditions, because of the difficulty in establishing the controller timing, the pulsator frequency may become in tune with the resonant frequency, and as a result, may actually increase the resonance of the chamber. If the pulsator can not be set to cancel or detune the resonant frequency, the pulsator is not utilized and an operating range of the combustor may be limited.